This invention related to the formation of a conductive layer of metal on a substrate. In particular, this invention relates to such a process wherein the conductive layer is formed from discrete metal particles of copper or nickel or combinations thereof. The invention also particularly relates to such processes wherein the metal layer is rendered conductive without the aid of a chemical agent.
It is frequently desired to form a layer of conductive metal on a (generally) non-conductive substrate. Such composites are useful for printed circuit boards, electromagnetic interference (EMI) shielding, and so forth.
Japanese publication JP110704(84) (Shin Gijutsu Kaihat) reports that a conductive layer may be formed on a substrate by spraying super fine (e.g.: 0.02 .mu.m) metal particles with the aid of a carrier gas (e.g.: nitrogen or argon) onto a substrate, followed by low temperature (e.g.: 80.degree. C.) sintering. One significant disadvantage of this process is the limited availability of such fine metal dust. Further, the use of metal dust in an air spray system requires extensive hygienic safeguards.
Japanese publication JP66133(73) (Fujimori Kogyo) and DE2,163,118 (Fujimori Kogyo (Sakai)) both describe a process where metal particles are mixed with a binder, applied to a substrate, dried, and treated with an agent which is an acid, a halogen, or a halogenide. Such processes are inconvenient and damaging to equipment in their requirement for a binder and a corrosive agent.
GB 2,085,340 teaches a paint containing copper particles and optionally a wetting agent (preferably triethanolamine), which is useful for producing conductive coatings.
U.S. Pat. No. 4,434,084 (Dupont (Hicks)) and DE 2,411,988 (Dupont (Beske)) both describe processes wherein a mixture of copper and tin particles are coated on a substrate with the aid of chemical activators and heat sufficient to melt the tin. In the U.S. patent the chemical activators are an organic acid flux and an organic amine in an inert organic medium. In the German publication the chemical activator is a flux which may contain triethanolamine as an adjuvant.
Applicants' co-pending prior application, titled Conductive Metallization of Substrates, serial number 068,593, filed June 30, 1987 now abandoned claims a method of forming a conductive layer, which method requires the use of a chemical developing agent.